Q&A: Snow on Batts

Q.I'm going to be removing the
existing fiberglass batts from the attic of a
moderately leaky older home, doing some air-sealing,
and re-insulating with 16 inches of loose-fill
cellulose. According to the homeowner, a dusting of
fine powdery snow sometimes covers the batts after
snowstorms, apparently because it sifts through the
ridge cap of the metal roof. This hasn't seemed to hurt
the batts any, but because cellulose is organic, I'm
wondering if these occasional dustings of snow could
lead to rot or mold.

A.Bill Hustrunk, technical
manager of cellulose manufacturer National Fiber,
in Belchertown, Mass., responds: The amount of
snow you describe won't do any harm. Cellulose is
highly hygroscopic, meaning it can disperse
moisture over a very large internal surface area
and then dry back out to reestablish its moisture
equilibrium. Because of this, there are no
localized wet areas that might give mold or other
microorganisms the moisture they need to multiply.
The borate-based fire retardants used in cellulose
help, too, because they have antimicrobial
properties.

The attic you'll be working in sounds
well-ventilated, but cellulose's ability to
accommodate moisture also allows it to work in
unvented dense-packed roof assemblies. Its density
prevents airflow - which we now know is the major
transport mechanism for moisture - while moisture
that enters through diffusion transport is managed
by its hygroscopic properties. But it's important
to note that this requires an assembly that's able
to dry to at least the inside or outside, and
preferably both.

During my nine years running the weatherization
program in central Vermont, of the thousands of
unvented roof assemblies that we dense-packed, I
saw only one failure - in a home where a large tree
limb fell on a roof during a rainstorm.